In situ observation of solidification behavior in undercooled Pd–Cu–Ni–P alloy by using a confocal scanning laser microscope

Abstract

In the undercooled melt of Pd 40Cu 30Ni 10P 20 alloy, the solidification behavior including the nucleation and growth of crystals at the micrometer level has been observed in situ by use of a confocal scanning laser microscope combined with an infrared image furnace. The Pd 40Cu 30Ni 10P 20 alloy specimens were cooled from the liquid state to various undercooled states under a helium gas flow. Images of solidification progress were obtained by the charge-coupled device image sensor of the confocal scanning laser microscope. Depending on the degree of undercooling, the morphology of the solidification front changed among various types: faceted front, columnar dendritic front, cellular grain and equiaxed grain, etc. The velocities of the solid–liquid interface were measured to be 10 −5–10−7 m/s, which are at least two orders of magnitude higher than the theoretical crystal growth rates. Combining the morphologies observed in the three undercooling regimes and their solidification behaviors, we conclude that phase separation takes place in the undercooled molten Pd 40Cu 30Ni 10P 20 alloy. The continuous-cooling–transformation (CCT) diagram was derived from the experimental time–temperature-transformation diagram constructed from solidification onset times under various isothermal annealing conditions. The CCT diagram suggests that the critical cooling rate for glassy solidification is about 1.8 K/s, which is in agreement with previous calorimetric findings.